Subsea well intervention vessel

ABSTRACT

A subsea well intervention vessel including a dynamically positionable tanker and direct well intervention equipment mounted on a deck of the tanker. The direct well intervention equipment is mounted on a superstructure above the main deck of the tanker and includes equipment for underbalanced non-rotating drilling and hydrocarbon liquid separation. The liquid separation equipment is coupled to storage tanks of the tanker so as to receive separated hydrocarbon liquids for storage purposes.

The present invention relates to a subsea well intervention vessel.

Hydrocarbon production wells are established by using a rotating drillassembly. A rotating drill assembly is driven from the surface,generally in the case of a subsea well from a rig mounted on a platformpositioned over the well. The platform can be mounted on the seabed ormay be a semi-submersible assembly the location of which can bemaintained in all but the most extreme conditions. After completion ofdrilling, the well is lined with tubing to enable hydrocarbon liquids toflow through the tubing from any hydrocarbon reserve into which thetubing extends. In some formations, hydrocarbon fluids and water occupythe same reservoir, the hydrocarbon fluids forming a layer on top of thewater. If the production tubing of a well penetrates the formationinitially occupied by the hydrocarbon fluids, as fluid flows to the welltubing the phenomenon known as “water coning” can occur, that is theinterface between the hydrocarbon liquids and water slopes upwardstowards the well. This effect results from pressure gradientsestablished within the reservoir formation as a result of fluid flowthrough the formation to the well tubing. If the tip of the cone-shapedinterface reaches the well tubing, large volumes of water will enter thewell tubing, reducing the rate of hydrocarbon liquid production andincreasing the costs of separating the produced hydrocarbon fluids fromthe water.

In wells where water coning has become a problem, it is known to conductfurther drilling operations so as to prevent or minimise water conegeneration. For example, a bottom hole drilling assembly can be used todrill lateral passageways into the hydrocarbon liquid-bearing formation.This can be achieved by using conventional drilling techniques, but suchtechniques demand the shutting down of the well and often require theremoval of the tubing lining the well. This involves substantial costsand risks. In addition, the hydrocarbon liquid bearing formation can bedamaged by drilling fluids required for the additional drillingoperations.

In order to avoid the possibility of loss or damage to a well resultingfrom drilling interventions, an advanced drilling technology has beendeveloped which allows technically difficult drilling to be achievedwithout substantial risk of damage to the formation. The technique isreferred to as “underbalanced” drilling. With underbalanced drilling,the well is live (positive pressure at the surface) at all times. Thiscan be achieved by either using a lightweight drilling fluid or relyingupon gas lift control using a purpose-built blow out preventer assembly.A clean drilling fluid is pumped down the well, and this mixes with theformation fluids that are allowed to flow up the well, that flowtransporting the rock cuttings to the surface. The five phases (gas,oil, formation water, drilling fluid and drilling solids) are thenseparated. On land this is a straightforward process as space is not ata premium. The equipment however is large and has not been thoughtsuited for offshore operations.

Underbalanced drilling can be conducted using either conventional rotarydrilling or coiled tubing drilling. In the UK sector of the North Seafour wells have been drilled using underbalanced rotary drilling butthis has only been possible using relatively large fixed(seabed-supported) platforms. On land, coiled tubing drilling has beenused. In these known applications, a long seamless pipe which is storedon a drum is pushed into the well by an injector against the live wellpressure. A turbine drill is mounted on the bottom end of the pipe andhydraulic pressure is delivered to the turbine drill through the pipe.This drives the turbine and permits drilling to take place. The smalldiameter of the pipe (typically 1 to 2 ⅞″) makes it possible for thepipe to pass through existing well-lining tubing (normally referred toas completions) so that it is not necessary to incur the substantialcosts and risks of removing such tubing.

Light intervention vessels are available which make it possible toconduct operations such as well servicing, e.g. well logging and generalmaintenance. Such vessels however cannot be considered appropriateplatforms for interventions requiring drilling as they are notsufficiently stable for such operations and furthermore could notoperate underbalanced drilling as they are too small to handle thevolumes of material that result in such drilling. Furthermore, lightintervention vessels require large capital investments as compared withthe returns that can be generated, particularly as they are highlyvulnerable to bad weather such that intervention costs are relativelyhigh and utilisation time is relatively low. It would of course bepossible to use a semi-submersible for well interventions butsemi-submersibles cannot be used as yet for underbalanced drilling. Evensuch an approach would require support vessels to receive the producedliquids and solids. Accordingly no attempts have been made to useunderbalanced coiled tubing drilling from floating units.

It is an object of the present invention to provide a subsea wellintervention vessel capable of re-entering existing production wells ina manner which allows well interventions to be performed withoutremoving the well from its production mode and without polluting thesubsea production system with well intervention effluent, e.g. drillingsolids.

According to the present invention, there is provided a subsea wellintervention vessel comprising a dynamically positionable tanker anddirect well intervention equipment mounted on the deck of the tanker,the direct well intervention equipment including equipment forunderbalanced non-rotating drilling and hydrocarbon liquid separationcoupled to storage tanks of the tanker such that separated hydrocarbonliquids can be stored in the tanker.

The invention also provides a method for conducting off-shoreunderbalanced drilling, wherein a tanker having direct well interventionequipment mounted on its deck is dynamically positioned over a riserextending from a subsea well, the well intervention equipment is coupledto the riser, and underbalanced non-rotating drilling is performed, theresultant multi-phase mixture being separated on the tanker andseparated hydrocarbon liquids being stored in storage tanks of thetanker.

The term “non-rotating drilling” is used herein to include any drillingin which there is no rotation of the drill string including but notlimited to underbalanced drilling using a rotary drill head poweredthrough a non-rotating drill string.

The well intervention equipment may be mounted on a superstructure abovethe main deck of a conventional shuttle tanker. Coiled tubing equipmentmay be mounted adjacent a skid deck which may be displaced to anoutboard position over a well riser to which the coiled tubing equipmentis to be connected. Thus a well intervention can be achieved bydynamically positioning the shuttle tanker adjacent a well riser, movingthe skid deck to the outboard position, coupling the coiled tubingequipment to the riser, and performing the necessary interventions inthe well to which the riser is connected, fluids and solids producedduring the coiled tubing drilling process being separated by equipmentmounted on the superstructure and hydrocarbon liquids being transferredfrom the separation equipment to the shuttle tanker storage hold.

As an alternative to providing a skid deck displaceable to an outboardposition, the drilling equipment could be mounted adjacent a moon poolextending through the tanker deck.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation taken from an available documentshowing the phenomenon of water coning;

FIG. 2 is a further illustration taken from a published document showingthe results of coiled tubing drilling in the structure of FIG. 1 so asto improve the rate of production of hydrocarbon liquids;

FIG. 3 is a side view of a known North Sea shuttle tanker incorporatingdirect well intervention equipment in accordance with the presentinvention;

FIG. 4 is a schematic layout diagram of the direct well interventionequipment shown in side view in FIG. 3; and

FIG. 5 is a schematic illustration of a tanker which defines moon poolsthrough which coiled tubing drilling can be performed;

FIG. 6 illustrates equipment for underbalanced non-rotating drilling andillustrates a hydrocarbon liquid separator.

Referring to FIG. 1, this illustrates a series of strata incorporating ahydrocarbon bearing stratum 1 which lies over a water bearing stratum 2.A well 3 is drilled through the strata 1 and 2. Pressure within thehydrocarbon liquid and water is such that flow is established to thewell 3. As a result of that flow a “water cone” 4 is defined around thewell 3 and as a result a conical interface 5 is established between thehydrocarbon liquid and water. If the well 3 is lined with steel tubingdown to the top of the strata 1, and the water cone reaches to adjacentthe lined portion of the well, large volumes of water will be produced.Clearly this is highly disadvantageous and therefore it is known tointervene in wells which suffer from the water coning effect. FIG. 2illustrates the results of such an intervention.

Referring to FIG. 2, a branch well 6 is shown as being drilled into thestratum 1. Drilling such a branch 6 can substantially improve theproportion of produced liquids made up by hydrocarbon liquids. It iswell known to form a branch such as the branch 6 of FIG. 2 using coiledtubing drilling techniques. It is necessary however when using suchtechniques to maintain underbalanced conditions (that is maintain apositive pressure at the top of the well 3) in order to avoid drillingsolids damaging the well. Such techniques have never been used offshorebecause the volume of material generated can only be handled in largeinstallations.

FIG. 3 illustrates a shuttle tanker embodying the present invention.FIG. 3 is based on a drawing extracted from “First Olsen Tankers” andshows a shuttle tanker of the type widely used in the North Sea. Theonly modification made to the standard shuttle tanker is the mounting ofa superstructure 7 above the main deck of the tanker, for example at aheight of approximately 3 m so as to clear the installed deck pipes andvents. On that superstructure all the equipment necessary for directwell intervention is mounted, including a crane 8. The detailed layoutof the equipment mounted on the superstructure 7 of FIG. 3 is shown inFIG. 4.

Referring to FIG. 4, a skid deck 9 is centrally mounted on thesuperstructure 7 adjacent a gantry crane 10. Coiled tubing drillingequipment 11 of conventional form is mounted adjacent the gantry crane10. A separator assembly 12 and ancillary drilling support equipmentassembly 13 are also mounted on the superstructure 7. All otherequipment relied upon to achieve the required direct well interventionis also mounted on the superstructure 7. The separator assembly 12 iscoupled to an appropriately positioned flare stack, for example at thestern of the vessel (not shown) and to the storage tanks of the tankerso as to enable produced hydrocarbon fluids to be stored for subsequenttransport.

In use, the tanker is dynamically positioned adjacent a subsea wellriser. The skid deck 9 is then moved to an outboard position (not shown)over the riser to enable the coiled tubing equipment 11 to be coupled tothe riser. Appropriate interventions can then be made via the riser andin particular coiled tubing drilling can be conducted in a manner whichproduces a multiphase mixture that is subsequently separated into itsdifferent phases in the separator assembly 12.

The system described with reference to FIGS. 3 and 4 represents abreakthrough in offshore drilling, testing, waste disposal and wellmaintenance. The tanker cargo holds can be used for the collection ofproduced oil during underbalanced drilling. The system can give directaccess to test subsea wells for extended durations. The system can beused for an extended water injection test and also allows for thedisposal of waste into a subsea well. Existing systems in contrastcannot perform coiled tubing drilling and cannot collect produced oil,requiring a separate shuttle tanker in the event that oil is beingproduced during drilling.

Furthermore the original features of the shuttle tanker are maintainedand therefore the vessel can still be employed in the charter marketwhen not being used for direct well interventions. As a result theinvention offers a solution to the problem of achieving direct wellinterventions with coiled tubing drilling without the major costsassociated with building and operating specialist vessels.

A standard North Sea specified shuttle tanker with dynamic positioningcan be readily chartered and fitted with a new deck above the installeddeck pipes and vents. On that deck appropriate equipment can beinstalled such as:

-   -   A skid mounted derrick riser handling unit with subsea control        panel;    -   Stumps for the subsea well intervention equipment;    -   A pipe rack;    -   Coiled tubing reels, control unit and power pack;    -   Cementing unit and blender;    -   Production test equipment including choke manifold, heater        treater, separators, degassing boot and gas flare;    -   Tanks for kill mud;    -   A closed circulation system for handling drilling mud and        drilled solids during underbalanced drilling;    -   Storage tanks for chemical and solid wastes;    -   Craneage for subsea equipment and supplies;    -   Remote controlled vehicles for working and observation tasks;    -   Water supplies for cooling and fire fighting services;

It is probably the case that there are of the order of 2000 subseacompletions currently operative. With the present invention, suchcompletions could be made accessible for of the order of 100,000 USdollars per day in contrast with currently quoted costs of the order of200,000 to 300,000 US dollars per day. Thus the invention dramaticallyaffects the technical capability of the offshore industry in the contextof the financial constraints which face that industry.

Coiled tubing drilling solutions include a cost-effective bottomassembly for standard mud systems and a wireline-based bottom holeassembly that fully exploits the benefits of through-tubing drilling,including use of foam and air systems. The present invention allowsonshore underbalanced drilling technology to be transferred offshorewithout requiring extended equipment development. It also permits theproduction of significant volumes of hydrocarbons without requiringadditional storage vessels, thereby reducing demands on cash flow whilstsimultaneously avoiding damage to a well as a result of drillingoperations. The motion characteristics of a relatively large shuttletanker are more suited for delicate underbalanced drilling operationsthen the available relatively smaller and more buoyant alternativevessels. This extends the amount of time that weather permits operationand reduces fatigue stress on the coiled tubing where it is fed from thetanker to the subsea well riser. The invention also allows wells to beproperly cleaned after interventions, thereby avoiding polluting thesometimes sensitive production system. Drilling waste can be managed inan optimal fashion, and all this can be achieved in relative safetygiven the large deck space available. All of these advantages areunavailable if using either a conventional semi-submersible vessel or aconventional purpose-built well intervention vessel.

In the embodiment of the invention described with reference to FIGS. 3and 4, components necessary for the operation of the invention aremounted on a skid deck which can be moved to an outboard position. In analternative arrangement illustrated in FIG. 5, such components aremounted adjacent moon pools extending through the structure of anotherwise conventional tanker.

Referring to FIG. 5, two moon pools 14A and 14B extend verticallythrough the structure of a modified shuttle tanker. Three cranes 15, 16and 17 can extend over the moon pools and areas indicating cargomanifolds 18, a derrick module 19, and a lay down area 20. Area 21houses gas compression and process units, area 22 a flare boom, area 23a flare knock-out drum skid, and area 24 a further lay down area servedby a crane 25.

Taking a standard double hull shuttle tanker, the modifications requiredto produce the vessel schematically illustrated in FIG. 5 which canfunction in accordance with the present invention would be an upgrade ofthe dynamic positioning capability, installation of a first moon pool (8m²) for intervention work, installation of a second moon pool (4 m²) forremotely operated vehicle work, mounting of cranes, process equipmentand lay down areas for deck-mounted equipment, and the mounting of flarefacilities and associated utilities.

FIG. 6 shows coiled tube drilling equipment comprising a seamless tube26 which is stored on a reel 27 and can be uncoiled from that reel andpushed into a well. The tube is passed through first and secondinjectors 28, 29 into riser 30 and through the riser into the well 31.The tube 26 supports on its end a bottom hole assembly 32 which includesa turbine drill powered hydraulically through the tube 26 from ahydraulic supply unit 33.

The system illustrated in FIG. 6 operates in an underbalanced drillingmode, that is the pressure in the hydrocarbon liquid-containingformation is greater than the pressure at the bottom of the well wheredrilling is taking place. This means that hydrocarbon liquid flows tothe well, that flow passing up the riser and conveying with it solidsproduced by the drilling process. The multi-phase mixture flowing up theriser is passed to a separator 34 where the hydrocarbon liquids areseparated from the mixture. Thus the pressure differential between thesurface (on the tanker deck) and the bottom of the well when drilling istaking place is less than the pressure at the bottom of the well whichin turn is less than the pressure at the bottom of the well which inturn is less than the pressure within the formation.

1. A subsea well intervention vessel comprising a dynamicallypositionable tanker and direct well intervention equipment mounted on adeck of the tanker, the direct well intervention equipment includingequipment for underbalanced non-rotating drilling and hydrocarbon liquidseparation coupled to storage tanks of the tanker such that separatedhydrocarbon liquids can be stored in the tanker.
 2. A vessel accordingto claim 1, wherein the well intervention equipment is mounted on asuperstructure above the main deck of a shuttle tanker.
 3. A vesselaccording to claim 1, wherein coiled tubing drilling equipment ismounted adjacent a skid deck which may be displaced to an outboardposition over a well riser to which the coiled tubing drilling equipmentis to be connected.
 4. A vessel according claim 1, wherein coiled tubingdrilling equipment is mounted adjacent a moon pool located over a wellriser to which the coiled tubing drilling equipment is to be connected.5. A method for conducting off-shore underbalanced drilling, wherein atanker having direct well intervention equipment mounted on its deck isdynamically positioned over a riser extending from a subsea well, thewell intervention equipment is coupled to the riser, and underbalancednon-rotating drilling is performed, the resultant multi-phase mixturebeing separated on the tanker and separated hydrocarbon liquids beingstored in storage tanks of the tanker.
 6. A method for conductingoff-shore underbalanced drilling, wherein a tanker having coiled tubingdrilling equipment mounted on its deck is dynamically positioned over ariser extending to a subsea production well, the coiled tubing drillingequipment including a non-rotating continuous coiled tube and ahydraulically driven drill mounted on one end of the tube, the coiledtubing drilling equipment is coupled to the riser and the tube isuncoiled and pushed through the riser into the production well so thatthe drill is located at a location where drilling is to be performed,hydraulic fluid is supplied to the drill through the tube to drive thedrill, drilling being underbalanced such that a multi-phase mixturewhich includes hydrocarbon liquids and solids is produced at the drilllocation which is at a pressure greater than the pressure differentialbetween that location and the tanker deck, the mixture is delivered tothe tanker through the well and the riser, hydrocarbon liquids areseparated from the mixture on the tanker, and the separated hydrocarbonliquids are stored in storage tanks of the tanker.